Surgical techniques for total knee arthroscopy (“TKA”) involve the measurement of the flexion and extension gaps. Soft tissue imbalance and bony misalignment can lead to malfunction and failure, thus the accurate measurement of the flexion and extension gaps is critical to a successful TKA procedure.
Surgical techniques for TKA typically begin with resection of the proximal tibia, and the balancing of the soft tissues of the knee in extension. The knee is then placed into flexion and a cutting block employed to obtain a rectangular flexion gap while the knee is distracted in flexion. After measuring the flexion gap, the distal femur is then resected. The knee is placed back into extension and the extension gap is measured and compared to the flexion gap. Additional resection of the distal femur is performed if required to ensure equal gaps.
The flexion and extension gaps should be as nearly equal as possible when the collateral ligaments are taut. Such tensioning is necessary because tendons and ligaments are relatively inextensible, and the spatial differences between the prosthetic knee and the natural joint may result in excessive tensile forces or excessive looseness. Tensioning the soft tissue while balancing flexion and extension gaps ensures stability and a full range of motion. If an implant is selected to fit the flexion gap and the flexion gap exceeds the extension gap, the knee will be incapable of full extension. Conversely, if the extension gap exceeds the flexion gap, the knee will hyper-extend and will be unstable in full extension. If an implant is selected to fit the extension gap, similar problems arise in flexion, as does the risk of dislocation
There are basically three existing surgical instrumentation systems for measuring flexion-extension gap. The first system employs solid blocks of different thickness called spacers. After making the preliminary bone cuts, a spacer of known thickness is inserted into the knee during flexion and extension. The surgeon typically distracts the knee and observes the soft tissues around the block to ensure they are balanced. If either the tension in the soft tissues or the flexion-extension gap is not balanced, then additional bone resection may be required. This system does not measure the tension in the soft tissue, but instead relies on the subjective perception of the surgeon and his ability to reliably and repeatedly distract the knee in flexion and in extension.
The second system employs a pair of distraction clamps, which are similar to large pliers. One clamp is used to distract the medial soft tissue while the other is used to distract the lateral soft tissue. With the soft tissue distracted, the flexion and extension gaps are measured either by additional instrumentation or with the distraction clamps if they provide means for measuring distances between bone surfaces. Distraction clamps are often quite large, particularly if they also perform measurement functions, and clamps have the tendency to damage the bone surfaces in which they make contact. Also, the surgeon must tension the clamps manually and ensure equal tension across the lateral-medial plane, which often proves to be a difficult process.
The third system employs a device known as a tensor designed to tense the knee during flexion and extension. After resection of the proximal tibia and the posterior femoral condyles and with the knee in flexion, the surgeon inserts the tensor into the flexion gap. The surgeon then operates the device by turning knobs or squeezing handles, which tensions the collateral ligaments. After applying a predetermined force, the device may be used to measure the flexion gap. The process is repeated with the knee in extension. The measured flexion gap is used as a guide to mark the distal femur at a location where the bone may be resected to provide an extension gap as near as possible to the measured flexion gap. The conventional tensor is not very practical due to the device's complexity and due to difficulties associated with its use.